usb: gadget: f_uac2: endianness fixes.

As per USB spec, multiple-bytes fields are stored
in little-endian order. Use CPU<->LE helpers for
such fields.

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>